The present invention relates to a power semiconductor switching device.
It has been known to use a thyristor, a gate turn-off thyristor and a transistor as a power semiconductor switching device.
The thyristor has not a self turn-off function. The gate turn-off thyristor is easily destroyed by the turn-off switching power whereby it is difficult to attain a large capacity. It is difficult to prepare a transistor having high voltage and large current because of the contradiction of the increase of breakdown voltage to the increase of current amplification factor.
It has been proposed to use a complex switching device wherein a gate turn-off thyristor is connected between the base-collector of the transistor whereby the transistor is turned off depending upon the turn-off caused by the gate of the gate turn-off thyristor.
FIGS. 16(a), (b) are respectively a sectional view of the conventional structure of the connection of the gate turn-off thyristor GTO and the transistor TR and the circuit diagram thereof.
The conventional structure comprises a three layer transistor part TR and a four layer gate turn-off thyristor GTO and the gate turn-off thyristor turns on and off the base current I.sub.B of the transistor.
Since the turn-off by the gate of the gate turn-off thyristor is difficult, it is necessary to increase the current amplification factor h.sub.FE of the transistor TR and to decrease the required base current I.sub.B for moderating the turn-off condition, because the current capable of turn-off of GTO by the gate, that is the OFF capable current per unit area of the semiconductor wafer (current density) is one order smaller than that of the thyristor which need not have the turn-off capability, and is one of several to that of the transistor.
Accordingly, the area of the semiconductor wafer for GTO is too large except lowering the current for passing through GTO and the required base current I.sub.B of the transistor.
In order to increase the current amplification factor h.sub.FE of TR, it is necessary to give thinner thickness of the base layer (13b) of TR part when the semiconductor having the same impurity is used.
However, the gate layer (13a) of the four layer GTO part should be thick because of the turn-off purpose and the four layer structure.
When their thicknesses are the same, the gate layer (13a) of GTO part and the base layer (13b) of TR part should have different impurity structures.
Such contradiction of the three layer TR part to the four layer GTO part is caused in the other semiconductor layers (12) (14, 14a, 14b).
Accordingly, the manufacture of the conventional element shown in FIG. 16 is complicated.
Moreover, in the conventional device of FIG. 16, it is difficult to increase the breakdown voltage of the three layer TR part. In order to improve the current amplification factor h.sub.FE, it is necessary to increase the diffusion distance of the base layer (13b) or to give thinner thickness or to give fine projected parts of the base contact.
These structures cause the breakdown voltage and the collector sustaining voltage V.sub.CE (sustaining) required for the turn-off step to decrease and cause higher nonuniformity in the layer, and an increase in the fault rate in the manufacture thereof.
When the breakdown voltage is increased (V.sub.CEO(sus) .gtoreq.600), the current density per unit area should be remarkably decreased. Accordingly, the high voltage-large current semiconductor element is remarkably uneconomical in comparison with a simple high speed thyristor. Accordingly, the high voltage semiconductor elements have been practically used when they are small current semiconductor element (several ampers).
The safety operation region is narrow because the collector sustaining voltage is low and the thickness of the base layer is thin to cause nonuniformity (high collector loss and tendency of local concentration of switching power).
The embodiment of FIG. 16 has the disadvantages of the transistor.
As stated above, the disadvantage of the embodiment of FIG. 16 is based on the fact that the four layer GTO part is turned off by only the gate. The three layer TR part does not substantially contribute in the turn-off operation as quality and contributes only for the amplification as quantity. Accordingly, when the current amplification factor of the three layer TR part is decreased, it is difficult to attain the turn-off by the gate of the four layer GTO part.
In the embodiment of FIG. 16, the current of TR is decreased depending upon the decrease of the current of GTO in the step of the turn-off of GTO, whereby the voltage across the main electrodes (41) and (42) is increased. Accordingly, the voltage rise is caused for decreasing the current. The turn-off switching power fed into GTO is not improved for GTO itself in comparison with the single GTO for the same current density (single GTO for the same current with the GTO part of FIG. 16) and there is only amplification by the TR part.
However, it has the fatal disadvantage as high power switching device that the turn-off switching power is locally concentrated in the principle of the turn-off by the gate.
For example, when gate reverse bias is applied in the turn-off step, the turn-off transient current i (off) is concentrated to the arrow line part separated from the gate electrode in FIG. 16 (a) during the time near the maximum switching power before and after the turn-off, that is, during the time of feeding the switching energy.
The embodiment of FIG. 16 has also the disadvantages of the conventional gate turn-off thyristor GTO.
As stated above, the turn-off capable base current is remarkably limited even though the amplification by the transistor TR is improved because of the function of the thyristor GTO itself. Accordingly, the large current should be given by improving the current amplification factor of the transistor TR.